Electric circuits



Aug. 7, I923. EAWIJH H. S. READ ELECTRIC CIRCUITS Filed Dec. 23, 1919 curreizi' //7 ve/vfor vbzzmye f/azr 5. Feed Patented Aug, 7, I923.

HARRY S. READ, OF NEW HAVEN, CONNECTICUT, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

ELECTRIC CIRCUITS.

Application filed December 23, 1919. Serial No. 346,992.

To all whom it may concern:

Be it known that I, HARRY S. READ, a citizen of the United States, residing at New Haven, in the county of New Haven, State of Connecticut, have invented certain new and useful Improvements in Electric Gircuits, of which the following is a full, clear, concise, and exact description.

This invention relates to electric circuits and more particularly it relates to methods for reducing distortion in amplifier circuits.

As is well-known in the art, weak currents may be repeated or amplified by impressing them upon the input terminals of a vacuum tube of the three electrode type and the amplified current in the output circuit of the tube will, in general, be a faithful copy of the impressed currents. In some instances, however, it has been found that the output circuit of the amplifier, while containing currents of theoriginal impressed frequency also contains in small amounts, harmonics of the original frequency. Inasmuch as these harmonics may, sometimes, be objectionable in the amplification of the impressed frequency or frequencies, the circuit arrangement of this invention has been provided to reduce these harmonics to a negligible value, if not entirely eliminating them.

In accordance with this invention, the undesired harmonics in the output current of a vacuum tube repeater or amplifier are substantially eliminated by associating with the amplifier a second vacuum tube repeater, in such a manner that harmonics enerated in one of the tubes are neutralized y harmonics generated in the other tube.

The manner in which this neutralization occurs will be better understood by reference to the following detailed description taken in connection with the accompanying drawing in which Fig. 1 represents this in vention embodied in a multi-stage amplifier e5 set of the vacuum tube type; Fig. 2 represents a characteristic curve for a vacuum tube amplifier; and Figs. 3 and d illustrate by curves how the arrangement of this invention serves to substantially eliminate harmonics produced by amplification.

The multi-stage amplifier set shown in the drawing comprises vacuum tube amplifiers 3 and 4: which are connected in tandem between incoming line 5, 5 and outgoing line 6, 6. Tube 3 contains a cathode 7, a control electrode 8 and an anode 9 while tube 4 contains a cathode 10, a control electrode or grid 11 and an anode 12. The two cathodes 7 and 10 are supplied with heating current from the battery 13. Output circuit voltage for the two tubes is derived from a battery 1: The drop in potential in high resistance 15 is impressed through a condenser 16 upon the potentiometer resistance 17, and thence upon the control electrode 11 of tube a. The amplified currents in the output circuit of tube 4 are impressed across resistance 22 and through a condenser 18 and resistance 19 upon the outgoing line 6, 6. With such an arrangement, incoming currents from line 5, 5 that are impressed across the terminals of resistance 20 will be amplified by tube 3. Tube 4 will amplify and impress on the outgoing line 6 the variation in the drop of potential in that part of resistance 17 located between the adjustable contact 21 and filament 10. 1

The manner in which harmonics generated in one of the tubes may be neutralized by harmonics generated in the other tube may now be explained as follows:

Suppose the special case, for purpose of illustration, that the incoming currents impressed across resistance 20 are of a single frequency which may be called the fundamental frequency or the first harmonic. The output current of tube 3 will contain the fundamental in amplified form and will also contain harmonics of the fundamental frequency in amounts depending in general upon the characteristics of the tube and the circuit constants. In most cases, however, the amplitude of the second harmonic will be greater than the amplitude of the third harmonic, the amplitude of the third harmonic will be greater than the amplitude of the fourth harmonic and so on. Assuming that the stages of the amplifier set are similar, adjustable contact 21 should be so adjusted that the input potential for the fundamental frequency impressed on tube 4 is the same as the potential of the fundamental frequency as impressed on tube 3 from line In other words, the contact 21 should be so adjusted ,that, as seen from tube 4:, the effective amplification of tube 3 is unity. The'amplified currents of the output circuit of tube l will, therefore, consist of the amplified fundamental, the amplified harmonics which were generated in tube 3,

and the harmonics generated bytube 4. As assumed above, the two stages 3 and 4 of the set are similar 50 that the harmonics generated by tube 4 will have the same intensity as the amplified harmonics which were generated by the first tube. Since the second, fourth and all even harmonics have the same direction, whether the input potential is positive or negative, and the first, third and all odd harmonics reverse when the input potential is reversed, it follows that in the output circuit of tube 4 the even harmonics generated by tube 4 will be equal and opposite in phase to the amplified harmonics which were generated by tube 3. This circuit arrangement, therefore, provides for the substantially complete elimination of the even harmonics produced by a vacuum tube repeater and since the second harmonic is often the harmonic that is present in the greatest intensity, this invention, therefore, serves to substantially eliminate the undesirable harmonics inYa repeated or amplified wave. y

The manner in which the even harmonics are substantially eliminated according to the arrangement of this invention will be more readily understood'by referring to Figs. 2, 3 and 4. Curve of Fig. 2 represents a portion of the usual output voltage space current curve of a vacuum tube amplifier. If it is desired in amplifying alternating currents by a vacuum tube to work over the range of the characteristic curve between points 31 and 32, it is obvious that harmonies will be generated because the characteristic curve is not straight between these two points. If the curve were straight between these two points, as indicated by the Y line 33, the output current of the amplifier would be an exact copy of the incoming waves, and if the incoming wave consisted of a single frequency, the output current would likewise com rise currents of a single frequency only. or purposes of illustration Figs. 3 and 4 have been drawn to indicate how the second harmonic is substantially eliminated by this invention. It will.

be understood, of course, that the other even harmonics will be eliminated in the same manner, although the curves shown in these two figures have not been drawn to show anythin but the fundamental frequency or the rst harmonic and the second harmomc. If we assume that the incoming current to amplifier 3 is of a simple sine wave and if we neglect the fact that the amplifier 3 will generate other harmonics beside the second armonic, the output current of tube 3 may be represented by curve 34 of Fig. 3. Such a curve may be resolved into its two components, namely the fundamental frequency curve 35, and the second harmonic curve 36. That the second harmonic should be in the phase shown in this figure will be apparent by noting that if the tube had a straight line characteristic as shown by line 33, the output current would be repre-' sented by curve 35. Due to the fact, however, that for the upper part of the working range the curve 30 is above the straight line 33, it'follows that due to the curved characteristic of the tube the output current for maximum positive voltages has a higher value than it should have for the generation of no harmonics. This ofcourse means that if the tube had a characteristic such as 33, the maximum output current would be of the value represented by the point 37, but

if the tube has a curved characteristic as 30,

its maximum output current would be greater, such as that represented by the point 38. In a similar manner for the second half cycle of wave 35 the tube gives an output current greater than it should give due to the fact that curve 30 for the lower working'range is above the straight line 33. If, now, a certain amount of the fundamental frequency wave 35 is tapped off and impressed upon the input circuit of tube 4, an alternating current wave of the same form as curve 35 will appear in the out-put circuit of tube 4, but 180 out of phase, due to the fact that an increase of current in the output circuit of tube 3 produces a decrease in the output circuit current of tube 4. The amplified fundamental wave is shown in Fig. 4 by the curve 40. The

amplification by tube 4 of an alternating,

current wave of the form shown by curve 35 will of course produce a second harmonic thereof which will be in the phase relation to curve 40 shown by curve 41 of Fig. 4. This of course is merely duplicating the relation of the second harmonic to its fundamental as shown in the fundamentals second half cycle of F 3. The input cir I .cuit of tube 4 also receives a portion of the second harmonic generated by tube 3 and represented by curve 36 of Fig. 3. For reasons noted above, this curve 36 will cause an alternatin current to appear in the output circuit 0 tube 4 of similar form but 180 outof phase. The amplified second harmonic generated by tube 3 is shown as curve 42 of Fig. 4. If, now, the adjustments are such as previously described, i. e. tubes 3 and 4 are similar and tube 3 is arranged to give an effective amplification of unity. curves 41 and 42 will be similar but opposite in phase, and will therefore neutralize each other. The output circuit of tube 4 will therefore not have currents therein of a frequency corresponding to the second harmonic of the alternatin current wave impressed u on tube 3. e same reasoning applies or the higher even harmonies that may be generated by the amplifging tubes of the set. It therefore follows t at this invention provides a means for substantially eliminating the even harmonics developed by the amplification of alternating current waves by amplifying elements having a curved characteristic.

This invention, however, is not limited to the case where the two stages are exactly similar. Even where the tubes have widely different characteristics the even harmonics may be considerably reduced by the adjustment of potentiometer 17 so that harmonics generated by stage 3 and amplified by stage t will be substantially neutralized by harmonics generated in stage 4.

It is to be understood that the circuit arrangement above described may be variously modified without departing in any wise from the spirit of this invention, as defined in the appended claims.

What is claimed is:

l. The method of amplifying an alternating current wave which comprises the steps of amplifying a wave whereby harmonics are produced, and amplifying such a portion of the amplified wave that harmonics generated by the first amplification are neutralized by the harmonics generated by the second amplification.

2. The method of neutralizing harmonics of an alternating current wave generated by the amplification of said wave by a multistage amplifier set, which method comprises substantially neutralizing the harmonics generated in one stage by the harmonics generated in another stage of said set.

of amplifying the alternating current wave whereby harmonics of the fundamental Wave are produced, reducing the amplified current wave to such an extent that the fundamental frequency is of the same intensity as it had prior to said amplification and re-amplifying the reduced current waves to an intensity equal to the intensity secured by said first amplification.

5. The method of repeating an alternating current wave which comprises the step of impressing said wave upon adevice havin an input voltage-output current curve characteristic whereby harmonics of said wave are produced, impressing said wave and said harmonics upon a second device having an input voltage-output current curved characteristic, and adjusting said devices to substantially eliminate certain of said harmonics.

In witness whereof, I hereunto subscribe my name this 20th day of December, A. D.

HEY S. READ. 

